Protection of a thin-layer battery by periodically operating battery at maximum discharge current

ABSTRACT

A method for protecting a thin-layer battery connected to an intermittent load including the steps of periodically operating the battery at its maximum discharge current, and disconnecting the battery as soon as the voltage across it reaches a threshold value greater than its critical voltage for the maximum discharge current.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patentapplication number 10/57104, filed on Sep. 7, 2010, entitled PROTECTIONOF A THIN-LAYER BATTERY, which is hereby incorporated by reference tothe maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for protecting a thin-layerbattery.

2. Discussion of the Related Art

The voltage across a thin-layer battery, as an example, of lithium-iontype, decreases along its operation to reach a critical voltage belowwhich the battery is irreversibly damaged. The battery discharge shouldthus be stopped and the battery should be recharged before it reachesthis critical voltage.

Thin-layer batteries used in systems with an intermittent operation, forexample, batteries powering self-contained sensors which periodicallycommunicate data (regularly or not) are here considered. In suchsystems, very short active periods alternate with inactive periods,which may be long. Further, the thin layer technology used for thebatteries necessitates that such batteries to have a strong internalresistance with respect to other batteries.

FIG. 1 is a diagram showing, versus time t, the current I provided by abattery powering such a sensor. Current peaks of high amplitude (forexample, 5 mA) and short duration, shorter than half a millisecond, areseparated by long periods, lasting for from a few seconds to severalhours, during which the current has a very small amplitude (for example,0.1 mA). Between times t0 and t1, the intermittent load is inactive. Theintermittent load is active between times t1 and t2 and between times t3and t4. The presence of a non-zero current during inactive periods isfor example due to the fact that, during these periods, the battery isused to power a low-consumption microcontroller.

Although systems for protecting batteries which detect the time whensaid batteries approach their critical voltage have been provided, aswill be discussed hereafter, such systems are not adapted to thin-layerbatteries with an intermittent operation.

SUMMARY OF THE INVENTION

An embodiment provides a method and a device for protecting a battery,adapted to a thin-layer battery connected to a load with an intermittentoperation.

An embodiment provides a method for protecting a thin-layer batteryconnected to an intermittent load comprising the steps of periodicallyoperating the battery at its maximum discharge current, anddisconnecting the battery as soon as the voltage thereacross reaches athreshold value greater than its critical voltage for said maximumdischarge current.

According to an embodiment, the periodic operation has a duty cyclesmaller than 0.1%.

According to an embodiment, the duration of each phase of the periodicoperation is shorter than 10 ms.

According to an embodiment, the period of the periodic operation isshorter than 10 minutes.

An embodiment also provides a device for protecting a thin-layer batteryconnected to an intermittent load comprising a switchable load capableof periodically operating the battery at its maximum discharge current,and a voltage comparator capable of comparing the voltage across thebattery with a threshold value greater than the critical voltage of thebattery for said maximum discharge current.

According to an embodiment, the switchable load is controlled by firstswitching means having a duty cycle smaller than 0.1%.

According to an embodiment, the first switching means are capable offorming a signal formed of square pulses having a width smaller than 10ms.

According to an embodiment, the first switching means are capable offorming a signal having a period shorter than 10 minutes.

According to an embodiment, the device comprises a buffer capacitor inparallel on the intermittent load.

According to an embodiment, the device comprises second switching meansarranged between the assembly comprised of the battery and of theswitchable load and the assembly comprised of the buffer capacitor andof the intermittent load.

According to an embodiment, the switchable load is a current source.

The foregoing and other objects, features, and advantages will bediscussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of the current provided by abattery powering an intermittent load;

FIG. 2 is a diagram showing the voltage across a battery according toits discharge rate for two discharge current values;

FIG. 3 illustrates an example of assembly of a battery powering anintermittent load provided with a system for protecting the battery;

FIG. 4 is a diagram similar to that of FIG. 2 which illustrates thevariation of the voltage across a battery connected as illustrated inFIG. 3;

FIG. 5A is a diagram showing an example of the current provided by abattery powering a specific intermittent load;

FIG. 5B is a diagram similar to that of FIG. 2 illustrating thevariation of the voltage across a battery providing the current of FIG.5A.

DETAILED DESCRIPTION

The present inventors have studied the behavior of a battery capable ofproviding alternately high and low currents and have searched forsolutions to optimize the use of such a battery.

FIG. 2 is a diagram showing voltage V across a specific lithium-ionbattery according to its discharge rate TD for two discharge currentvalues, respectively 0.1 and 5 mA. The discharge rate is zero when thebattery is fully charged. It increases along the battery operation andshould never reach a maximum value, to avoid irreversibly damaging thebattery. For a 0.1-mA discharge current, the voltage decreases from aninitial 4.2-V voltage to a critical 3.6-V voltage V_(C1). For a 5-mAdischarge current, the voltage decreases from an initial 3.75-V voltagedown to a critical 3-V voltage V_(C2).

Thus, when the battery is connected to an intermittent load with acurrent varying as illustrated in FIG. 1, the operating point followscurve C1 in inactive periods of the load and follows curve C2 in activeperiods. Given that the battery may have long inactive periods, it ispossible for the operating point to constantly move along curve C1 allthe way to point 1 of this curve corresponding to a voltage V_(TH1)slightly greater than voltage V_(C1). To avoid the battery entering anirreversibly damaged state, said battery should be disconnected as soonas point 1 has been reached. The voltage across the battery should thusbe permanently compared with a threshold voltage V_(TH1) slightlygreater than V_(C1) and the battery should be disconnected as soon asthe threshold has been reached.

The choice of a threshold V_(TH1) slightly greater than V_(C1) has amajor disadvantage for the battery when operating at its maximumdischarge current. If, in active periods of the load, the batterydischarges from a point 3 of curve C2, as soon as a point 4corresponding to voltage V_(TH1) is reached, the battery isdisconnected. Similarly, if the battery is at an operating point 5 ofcurve C1, and the load causes a current surge which should shift theoperating point to point 6 of curve C2, the battery is disconnectedsince the voltage across the battery, for point 6, would be lower thanV_(TH1). Thus, for an operation at maximum current, the battery isdisconnected at the level of point 4 while it still contains a largeamount of power. Point 7 of curve C2 corresponding to a voltage V_(TH2)slightly greater than critical voltage V_(c2) can never be reached forthis operating mode.

FIG. 3 illustrates an example of assembly of a battery powering anintermittent load provided with a battery protection system. A battery11 powers an intermittent load 12. A voltage comparator 13 compares thevoltage across the battery with a threshold V_(TH). This comparator isconnected to the control terminal of a switch SW1 having a firstterminal connected to battery 11 and a second terminal connected to anode 15. Node 15 is connected by a switch SW2 to a load 16, calledswitchable load hereafter, and by a switch SW3 to a first terminal of abuffer capacitor 17 and to load 12.

The assembly illustrated in FIG. 3 is provided to operate according toone or the other of two modes. These two modes periodically follow eachother until battery 11 is disconnected.

In a first mode, switches SW1 and SW3 are on and switch SW2 is off. Ininactive periods of load 12, battery 11 briefly charges buffer capacitor17, and then powers a microcontroller (not shown) under a 0.1-mAcurrent. In active periods of load 12, battery 11 and buffer capacitor17 power load 12, the capacitor being sized to provide most of the peakcurrent to the load and to limit the current peak required from thebattery (here to 5 mA). The strong internal resistance of the batterywould prevent it from providing a stronger current.

In a second so-called forced operating mode, switch SW1 is on, switchSW2 is turned on for a short time, and switch SW3 is turned off for ashort time. Switchable load 16 forces the battery to operate at itsmaximum 5-mA discharge current. In an active period, load 12 remainspowered by buffer capacitor 17.

FIG. 4 is a diagram similar to that of FIG. 2 which illustrates thevariation of the voltage across a battery connected in an assembly ofthe type in FIG. 3, with an intermittent load 12 having an activitycorresponding to the current illustrated in FIG. 1.

At an initial time t0, battery 11 is fully charged and the voltagethereacross is 4.2 V. Indeed, the discharge current is 0.1 mA since load12 is not active, as illustrated in FIG. 1. Switches SW1 and SW3 are on,and switch SW2 is off. Capacitor 17 charges and the voltage across thebattery decreases along curve C1.

At a time t11, switch SW2 is turned on for a short time while SW3 isturned off. Switch SW1 remains on. Load 16 forces battery 11 to operateat 5 mA. The voltage across battery 11 drops vertically from point A ofcurve C1 to point B of curve C2. It is then shifted along curve C2 frompoint B to point C corresponding to a time t12. At time t12, switch SW2is turned off and SW3 is turned on. Load 12 being inactive, the value ofthe discharge current of battery 11 varies from 5 mA to 0.1 mA, frompoint C of curve C2 to point D of curve C1.

This short round trip between curves C1 and C2 of the voltage across thebattery is repeated at times t13 and t15. Times t11, t13, and t15 arepreferably regularly spaced apart. Time intervals t11 to t12, t13 tot14, and t15 to t16, during which load 16 forces the operation ofbattery 11 to a 5-mA discharge current, are short and preferably equal.

At time t1, defined in FIG. 1, intermittent load 12 becomes active. Thecurrent provided by battery 11 increases from 0.1 mA to 5 mA. Theoperating point shifts from point E of curve C1 to point F of curve C2.Between times t1 and t2, the current provided by the battery is equal to5 mA. The operating point shifts from point F to point G of curve C2. Attime t2, the current provided by battery 11 decreases from 5 mA to 0.1mA. The operating point shifts from point G to point H of curve C1.

Since load 16 periodically forces the operation of the battery to a 5-mAdischarge current, iterations of this forced operation are likely tooccur between times t1 and t2. In the shown example, a first iterationsubstantially occurs at time t1 and ends at a time t17, and a second onestarts at a time t18 and ends at a time t19. As seen previously, suchiterations have no influence on the power supply of load 12 due to thepresence of buffer capacitor 17.

After time t2, load 12 remains inactive, the battery operating pointmoves along curve C1 except during the time intervals from t20 to t21and from t22 to t23. Such time intervals correspond to two iterations ofthe forced operation.

At a time t24, a new iteration of the forced operation starts. Battery11 then attempts to operate at a 5-mA discharge current. The operatingpoint cannot pass from a point I of curve C1 to a point of curve C2without the voltage across the battery becoming lower than a voltageV_(TH2) slightly greater than V_(C2). The battery discharge isinterrupted by the turning off of switch SW1 as soon as the voltageacross the battery is equal to voltage V_(TH2).

It should be noted that, in our example, that is, thin-layer batteriesfor self-contained to sensors, a system for recovering power (heat,vibration, radiation, light) intermittently charges the battery (whenthe power source is available). This system (not shown herein)automatically reconnects the battery which has secured itself at timet24, when the power source is available again.

This automatic reconnection does not endanger the battery discharged attime t24 since the power supply can only raise its voltage and draw itaway from threshold V_(TH2).

It is thus provided herein to submit the battery to repeated iterationsof high-current operation and to select, as threshold voltage V_(TH), avalue V_(TH2) slightly greater than critical voltage V_(C2)corresponding to an operation with a high current. This choice of athreshold V_(TH2) slightly greater than V_(C2) is very advantageous forthe battery when operating at its maximum discharge current. Indeed, thebattery is disconnected at operating point 7 only, that is, when all thepower available in the battery for a 5-mA current has been used.

The periodicity of the forced operation is selected to be short, so thatabove-mentioned point I is not very distant from point 8 of curve C1,located vertically above point 7 of curve C2, and so that point I isvery distant from the end point of curve C1 corresponding to thecritical voltage at low current. In the example illustrated in FIG. 4,the battery can provide a 0.1-mA current for more than one hour when theoperating point moves along curve C1 from point 8 to the end point. Thisend point does not risk being reached if the periodicity of the forcedoperation is selected to be much shorter than one hour, for example, onthe order of some ten minutes.

Further, the duration of the forced operation will be selected to beshort, to minimize the electric consumption induced by the manyiterations of the forced operation.

FIG. 5A shows an example of the variation of current I along time t,linked to the coupling of a battery and of a specific intermittentsource.

Between times t30 and t31, the load is inactive. Starting from time t31,five current pulses of a duration of 0.2 s each are separated by 0.3 s.Then, until a time t32 equal to t31 plus two hours, the current remainslow. At time t32 and at subsequent times t33=t32+2 h and t34=t33+2 h,new groups of five pulses identical to those corresponding to time t1start. Between the 5-mA current pulses, the current is equal to 0.1 mA.

FIG. 5B is a diagram similar to that of FIG. 2 which illustrates thevariation of the voltage across a battery connected to the intermittentload for which the current is described in FIG. 5A.

Between times t30 and t31, the battery operating point shifts from apoint 21 to a point 22 of curve C1 corresponding to the time.

The operating point then tends to shift from point 22 of curve C1 to apoint 23 of curve C2.

With the method wherein the threshold voltage is chosen to be equal toV_(TH1), it is impossible to reach point 23, which is to the right ofpoint 4 described in relation with FIG. 2. The battery is disconnectedat point 23 without ever having powered the intermittent load, while itstill contains energy. For the battery to power the intermittent loadwith a 5-mA current, time t1 would have had to come before a time tm1corresponding to point 4 of curve C2 for which the voltage across thebattery is equal to V_(TH1).

However, if forced operation phases and a threshold voltage equal toV_(TH2) are provided as described hereabove, the battery operation cancarry on. The battery operating point performs five round trips (notshown) between curve C1 and curve C2 corresponding to the five 5-mAcurrent pulses. Then, the battery operating point follows curve C1 untilit reaches a point 24 corresponding to time t32. At times t32 and t33,the battery operating point performs five other round trips betweencurve C1 and curve C2. At time t4, the battery operating point cannotpass onto curve C2 since it is then far to the right of above-mentionedpoint 7 and the battery will have been disconnected.

In this case, the battery powers the intermittent load for 3 groups of 5current pulses. The battery is only disconnected when all the poweravailable for a 5-mA current has been used.

Of course, the present invention is likely to have many variations.

For example, the battery is not necessarily a lithium ion battery, oreven a thin-layer battery. The battery protection system provided hereinapplies to any battery having a high internal impedance and dischargingas illustrated in FIG. 2, and which has a critical voltage below whichit can be irreversibly damaged.

It is possible not to immediately disconnect the battery when thethreshold value is reached. It may for example power an audio or visualwarning indicating that it should be recharged.

The switchable load may be a current source, and more specifically acurrent mirror.

The buffer capacitor, which has the function of powering theintermittent load during the forced operation and of limiting the valueof the maximum discharge current of the battery during active periods ofthe intermittent load, may have a capacitance of a few hundreds of tomicrofarads. This buffer capacitor is optional if no iteration of theforced operation coincides with active periods of the intermittent load,and if the current peak surged by the load does not exceed that whichcan be provided by the sole battery. It may also be replaced with anypower source having the same function.

The value of the maximum discharge current, as well as the duration andthe periodicity of the forced operation will be chosen by those skilledin the art according to the desired performance of the batteryprotection system.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A method for protecting a thin-layer batteryconnected to an intermittent load comprising the steps of: periodicallyoperating the battery at its maximum discharge current, anddisconnecting the battery as soon as the voltage across it reaches athreshold value greater than its critical voltage for said maximumdischarge current.
 2. The method of claim 1, wherein the periodicoperation has a duty cycle smaller than 0.1%.
 3. The method of claim 1,wherein the duration of each phase of the periodic operation is shorterthan 10 ms.
 4. The method of claim 1, wherein the period of the periodicoperation is shorter than 10 minutes.
 5. A device for protecting athin-layer battery connected to an intermittent load comprising: aswitchable load capable of periodically operating the battery at itsmaximum discharge current, and a voltage comparator capable of comparingthe voltage across the battery with a threshold value greater than thecritical voltage of the battery for said maximum discharge current. 6.The device of claim 5, wherein the switchable load is controlled byfirst switching means having a duty cycle smaller than 0.1%.
 7. Thedevice of claim 6, wherein the first switching means are capable offorming a signal formed of square pulses having a width smaller than 10ms.
 8. The device of claim 6, wherein the first switching means arecapable of forming a signal having a period shorter than 10 minutes. 9.The device of claim 5, comprising a buffer capacitor in parallel on theintermittent load.
 10. The device of claim 9, comprising secondswitching means arranged between the assembly comprised of the batteryand of the switchable load and the assembly comprised of the buffercapacitor and of the intermittent load.
 11. The device of claim 5,wherein the switchable load is a current source.
 12. A method foroperating a battery connected to an intermittent load, comprising:supplying current to the intermittent load during a normal operatingmode; supplying a maximum discharge current from the battery to aswitchable load periodically during a forced operating mode; anddisconnecting the battery from the intermittent load when a voltage ofthe battery drops to a threshold value greater than a critical batteryvoltage at the maximum discharge current.
 13. A method for operating abattery as defined in claim 12, wherein supplying current to theintermittent load comprises supplying current to the intermittent loadfrom the battery and from a buffer capacitor when the forced operatingmode is inactive.
 14. A method for operating a battery as defined inclaim 13, wherein supplying current to the intermittent load comprisessupplying current to the intermittent load from the buffer capacitorwhen the forced operating mode is active.
 15. A method for operating abattery as defined in claim 12, wherein the normal operating mode andthe forced operating mode overlap in time.
 16. A method for operating abattery as defined in claim 12, wherein a duty cycle of the forcedoperating mode is less than 0.1%.
 17. A method for operating a batteryas defined in claim 12, wherein a duration of the forced operating modeis less than 10 milliseconds.
 18. A method for operating a battery asdefined in claim 12, wherein a period of the forced operating mode isless than 10 minutes.
 19. A method for operating a battery as defined inclaim 12, adapted for operation of a thin-layer battery.
 20. A methodfor operating a battery as defined in claim 12, adapted for operation ofa lithium ion battery.
 21. A device for protecting a battery connectedto an intermittent load, comprising: a switchable load configured toperiodically operate the battery at a maximum discharge current in aforced operating mode; a comparator configured to compare a voltage ofthe battery with a threshold value greater than a critical batteryvoltage at the maximum discharge current; and a first switch configuredto disconnect the battery from the intermittent load when the comparatordetects that the battery voltage has crossed the threshold value.
 22. Adevice for protecting a battery as defined in claim 21, furthercomprising a buffer capacitor coupled in parallel with the intermittentload.
 23. A device for protecting a battery as defined in claim 22,further comprising a second switch in series with the switchable loadand a third switch coupled between the switchable load and theintermittent load.
 24. A device for protecting a battery as defined inclaim 21, wherein the switchable load comprises a current source.
 25. Adevice for protecting a battery as defined in claim 21, wherein a dutycycle of the forced operating mode is less than 0.1%.
 26. A device forprotecting a battery as defined in claim 21, wherein a duration of theforced operating mode is less than 10 milliseconds.
 27. A device forprotecting a battery as defined in claim 21, wherein a period of theforced operating mode is less than 10 minutes.
 28. A device forprotecting a battery as defined in claim 21, configured to protect athin-layer battery.
 29. A device for protecting a battery as defined inclaim 21, configured to protect a lithium ion battery.
 30. A device forprotecting a battery as defined in claim 23, wherein the second switchis closed and the third switch is open when the switchable loadperiodically operates the battery at the maximum discharge current.